Project Summary Abstract Thorp/Luo The research described in this proposal will uncover the mechanistic basis and therapeutic potential for new targets for cardiac transplant. Recent findings reinforce the hypothesis that graft ischemia reperfusion injury/IRI is an independent risk factor for a more complicated clinical course. Specifically, increased cardiac troponin levels 24 hours after cardiac transplant and reperfusion are directly linked to chronic graft failure and vasculopathy. A critical component of reperfusion injury is the innate immune response, which includes monocytes and macrophages (M?s). Cutting edge findings reveal novel molecular links between innate M?s, and cells of the post-transplant chronic and adaptive immune response. This proposal will test the overall hypothesis that the consequences of innate M? action during perioperative cardiac allograft IRI, specifically govern subsequent alloreactivity, vasculopathy, and tolerance mechanisms by Myeloid derived suppressor cells (MDSCs). Critical regulators of both inflammation and tolerance are the TAM receptor tyrosine kinases/RTKs. TAMs, particularly MERTK, promote actin-driven apoptotic cell phagocytosis, or efferocytosis, by M?s. Independently, the tyrosine kinase domain of MERTK is also capable of transducing intracellular phospho-relay signals to suppress immune cell-activation and potentially, promote immune-tolerance. Our preliminary data suggest MERTK is antagonized by two mechanisms: MERTK destruction by ADAM proteases, and intracellular signaling by another TAM family member, i.e., AXL. We hypothesize that MERTK-mediated efferocytosis mitigates perioperative graft IRI, the consequences of which are linked to subsequent promotion of allograft tolerance. We hypothesize suboptimal perioperative allograft preservation, ADAM proteases, and AXL antagonize beneficial MERTK actions. In addition to testing these hypotheses, we will also test their therapeutic potential and human relevance.